A typical work clamping assembly comprises a swinging clamping arm, actuated for example by toggle means, for moving a screw threaded plunger or toggle bolt generally axially in a clamping action against a workpiece. The screw threads of the bolt or plunger are cooperable with threaded locking means screwed thereon for selectively adjusting the effective axial distance along the plunger between the operating arm and a clamped workpiece.
The work engaging end of the plunger comprises an elastomeric bumper or cushion, commonly a rubber-like material such as neoprene, that engages and protects the clamped workpiece. The plunger preferably comprises a threaded steel rod or bolt protected by a metallic coating such as a copper or zinc dichromate plating and having an enlarged hexagonal head. The neoprene bumper is usually formed by compression molding on the bolt head which is encased within the bumper as an insert during the molding operation, thereby to interlock the bumper and bolt and also to provide a comparatively large area for distributing the clamping force against the neoprene bumper. Where the cross-sectional area of the bolt is sufficiently large and the clamping force required is sufficiently small, the enlarged bolt head may not be required, in which case the portion of the bolt embedded within the neoprene bumper may comprise a screw threaded stud. In any event an appreciable axial extent of the bolt must be chemically bonded to the bumper in order to withstand side loading or radial forces exerted by the bolt on the bumper during a clamping operation.
In the latter regard, when the bolt is carried by the swinging end of a toggle actuated clamping arm, the clamping movement of the bolt and bumper against the workpiece is slightly arcuate rather than truly axial. In consequence, the subassembly of the bolt and bumper are subjected to severe distorting forces that are unique to such assemblies. The bumper must be sufficiently elastic and resiliently deformable to serve properly as a protective cushion for the workpiece, yet must be sufficiently resistant to deformation and shearing to prevent its being punctured by the bolt during repeated use, such that the workpiece would be damaged by direct contact with the steel bolt. Also the bumper must cling to the bolt with sufficient tenacity to prevent relative radial displacement of the bolt that would eventually cause shearing and destruction of the material of the bumper. For these reasons, the enlarged bolt head bonded within the bumper and an appreciable axial extent of the bolt chemically bonded to the material of the bumper have been required heretofore. Otherwise a bumper sufficiently hard and form sustaining to cling to the bolt would be too hard and inelastic to withstand disintegration by the severe clamping forces and would not provide adequate cushioning for the workpiece.
The above noted limitations on conventional neoprene toggle bolt and bumper subassemblies give rise to several objections. At the outset, when one end of the threaded bolt is located within the mold as an insert when the bumper is molded, some of the moldable material runs along the screw threads of the bolt and hardens thereat as flashing. The effective unobstructed axial length of the threaded bolt is thus decreased and its axial adjustability with respect to the clamping arm of the toggle clamp is likewise decreased.
Furthermore, when the toggle bolt and bumper are molded together as a unit, difficulty is often encountered when the protective metallic plating is applied to the bolt. Federal regulations, for example, prohibit molding of the neoprene bumper on the preferred copper plated bolt. Accordingly the bolt must be copper plated after the molding operation. The plating procedure required for optimum copper plating reacts with the neoprene material of the bumper. Either a less satisfactory copper plating that does not react with the neoprene, or a less satisfactory substitute metallic plating such as the zinc dichromate plating must be accepted.
In addition, when the elastomeric bumper is molded on the toggle bolt and is integrally bonded chemically thereto, if the resulting article is defective when taken from the mold, both the bolt and bumper must be discarded. Because it is too difficult to remove the defective bumper from the bolt, the material of the bumper along with the bolt become a costly waste.
An important object of the present invention is to provide an improved toggle bolt and elastomeric bumper of the general character described which avoid the above objections and achieve a number of important advantages over similar conventional devices.
Another object is to provide such a toggle bolt and bumper wherein the bumper is molded from a thermoplastic polyurethane independently and separately from the bolt. The bumper is formed to provide a body of the polyurethane axially spacing a work abutting surface and a bolt abutting surface. The latter comprises the base of a socket adapted to receive one end of the bolt snugly therein with an interference fit. The axially extending walls of the socket comprise a support that resiliently engage and hold the bolt in coaxial alignment with the bumper when the two are assembled.
The bolt is likewise formed separately from the bumper and is thereafter removably assembled with the bumper as described herein by being forced axially endwise into the bolt receiving socket of the bumper until the inner end of the bolt firmly abuts the socket base. By virtue of the toughness of the polyurethane material of the bumper and its far greater resistance to shearing and permanent deformation as compared to neoprene or rubberlike materials of comparable elasticity or cushioning ability, the polyurethane bumper firmly grips the bolt and adequately resists the axial clamping forces without recourse to an enlarged forcedistributing bolt head, and similarly resists lateral or radial forces that tend to move the work engaging surface of the bumper laterally with respect to the bolt axis without necessitating a chemical bond between the bumper and bolt. At the same time the resiliency of the polyurethane provides the desired cushioning properties for protecting the clamped workpiece.
The polyurethane material of the bumper enables the latter and the bolt to be feasibly formed separately from each other and removably assembled together as desired, thereby to achieve several important cost savings as described below. Also by avoiding the necessity of molding and bonding the bumper on the toggle bolt, the bolt may be conveniently plated with any desired material and by the optimum plating process. In consequence a preferred high quality copper plating is readily feasible.
The elimination of an enlarged bolt head molded within the bumper, as required heretofore in order to obtain a bumper that would provide both the protective cushioning for the workpiece and also the load bearing quality necessary to survive the severe clamping force, also enables simplification of the mold and the molding operation because the mold in accordance with the present invention is not required to accommodate the bolt inserted thereinto. Likewise, by molding the bumper separately from the bolt, no flashing results along the bolt threads. A sharp demarcation between the bolt and the material of the bumper is thus obtained which is aesthetically pleasing and which renders the full axial length of the bolt exteriorly of the bumper available for axial adjustment with respect to the swinging clamping arm that carries the bolt.
Also by virtue of the toughness of the polyurethane material, not only is a chemical bond between the bumper and bolt rendered unnecessary, but a simple effective mechanical connection or interlock between the bumper and bolt is made readily feasible, whereby the bumper may be simply and economically assembled with the bolt and removably secured thereto.
The interlock may merely comprise a small radial enlargement in the bolt receiving socket and a mating radial enlargement swaged on the bolt, or may simply comprise the bolt threads effecting an interference fit with the circumferential wall of the socket, whereby the resilient polyurethane material yields without rupturing or shearing to enable insertion of the bolt into the socket, and thereafter resiliently flows into the thread grooves of the bolt to effect the necessary interlock. A partial screw turn of the bolt may then be applied to force the latter tightly against the bottom or base of the socket and thereby resiliently stress the polyurethane material to enhance the interlock.
In a preferred construction, the interlock comprises a couple of screw thread turns molded into the socket adjacent its base to mate with the threads of the bolt. In contrast to the rubberlike or neoprene material employed heretofore, the tough polyurethane material of the bumper will effect the necessary interlock if its threaded portions extend axially only a few helical turns from the base of the socket. The remaining major axial portion of the socket periphery or sidewall may be smooth and unthreaded and dimensioned to receive the threaded bolt snugly and slidably therein with a slight interference fit. Accordingly the bolt, which may comprise a steel shank threaded throughout its axial length, may be forced coaxially without rotary motion into the open end of the socket and substantially to the base of the latter before encountering the threaded portion of the socket. A couple of turns of the bolt within the socket will then screw the bolt to its assembled position with its end seated firmly against the base of the socket and with the polyurethane threads of the bumper resiliently stressed to effect the desired interlock.
Where it is convenient to assemble the bolt and bumper at the molding site for the bumper, the bolt may be pushed axially into the bumper socket entirely to the bolt abutting base without any screw action if done shortly after the bumper is removed from the mold, for example within about thirty minutes. During that time the polyurethane will not be completely cured and its threaded portion within the socket will yield to enable the insertion of the bolt by axial movement only, thus further simplifying the assembly. As described above, the bolt will be given a final partial screw turn to screw it firmly against the socket base and enhance the interlock.
Usually one and one-half or two turns of the screw threads molded within the socket are adequate to secure the bolt and bumper together and to withstand the shearing forces of a clamping operation. The slight interference fit between the unthreaded socket sidewall and threaded bolt will cause the unthreaded sidewall to flow into the thread grooves of the bolt. The toughness of the polyurethane will resist cutting or permanent deformation by the bolt threads.
In accordance with the structure described herein, if a molded bumper is defective, it may be recycled before being assembled on the bolt. If a defect in either the bolt or the bumper is noted after the assembly, the bumper can readily be unscrewed from the bolt and recycled. The saving in material will partially offset its cost. Additional savings can also be made in shipping costs when the molding site for the bumpers is remote from where the bolts are manufactured. There is no need to ship the comparatively heavy bolts to the molding site, then ship the assembled bolts and bumpers for assembly with the remainder of the clamping assembly. Instead, the lightweight bumpers can be shipped directly from the molding site to the final assembly.
Prior art known to applicant that should be made of record in this application are U.S. patents to:
Brown--U.S. Pat. No. 2,519,107--Aug. 15, 1950 PA0 Higgins--U.S. Pat. No. 2,930,409--Mar. 29, 1960 PA0 Kennedy--U.S. Pat. No. 2,379,529--July 3, 1945 PA0 Ferguson--U.S. Pat. No. 2,551,834--May 8, 1951 PA0 Wilson--U.S. Pat. No. 2,705,336--Apr. 5, 1955
Brown shows a clamping device comprising a threaded bolt 15 and cushioning element of neoprene or rubber-like material secured to an enlarged head 14, which the present invention specifically avoids. Also the cushion 18 of Brown is not subject to a radial clamping force and provides no means for withstanding such a radial force, as for example a supporting socket comparable to applicant's for confining the threaded bolt shank coaxially therein with an interference fit throughout an appreciable axial extent.
The last four patents are typical of protective caps for various purposes. None suggests taking advantage of the properties of a molded polyurethane material in combination with a specific structure comprising a bolt removably secured within the socket of a polyurethane bumper and resiliently gripped by an interference fit with the socket wall throughout an axial extent greater than the bolt diameter, such that without recourse to chemical bonding between the bolt and bumper or an enlarged force distributing bolt head, the bumper readily withstands the unavoidable axial and radial clamping forces without being crushed or sheared by the bolt and also effects a highly resilient contact with the clamped workpiece, whereby the bumper is readily and economically replacable when worn, the bolt may be readily coated with any desired metallic plating by the most effective process available, and the above noted advantages and economies resulting from molding the bumper separately from the bolt are obtained.